In digital radiographic imaging systems a source projects a beam of x-rays toward a subject or object under study, such as a patient or a piece of luggage. After being attenuated by the subject or object, the beam impinges on multiple radiation detector elements, each of which produces an electrical signal indicative of beam attenuation. The x-ray detector system typically includes a collimator disposed above the sensing elements for limiting the spread of the x-ray beam before it traverses the sensor media in which x-rays are converted to electrical signals. Typically the detector elements are uniformly spaced apart in a series of arrays along an imaging plane. Resulting electrical signals are processed to render an image formed of discrete pixels, with each pixel having a brightness level based on the signal received from a detector element. Nominally, the limiting spatial resolution of the image formed in a radiographic imaging system is set by the width of the detector elements.
In contemporary computed tomography (CT) systems, the x-ray source and the array of detector elements are rotated on a gantry and around the subject. The CT image is formed by a reconstruction algorithm applied to data acquired on multiple views at different gantry angles. Spatial resolution of the CT image is a function of the detector element size, as a smaller detector size enables an increased number of detector elements per unit area and improved image resolution. However, the relative cross-talk across element boundaries increases as the element size decreases.
Improved spatial resolution of the CT image may also be had by reconstructing an image based on data collected at first and second detector array positions with a complementary arrangement. The second position may be slightly offset with respect to the first position, i.e., offset by a fraction of the spacing between adjacent detector elements. That is, such offset pixel imaging, providing a higher sampling of the incident radiation, renders greater spatial resolution in the reconstructed image. It is typical for a CT system to implement this offset by a quarter-element angular offset of the detector centerline in the gantry system. Data acquired in views 180 degrees (one-half rotation) apart are then complementary with half-element offset.
In an indirect conversion detector system, x-rays are first converted to light in scintillation elements. The scintillation elements are packaged in an array with reflective bonding materials between the elements. Light-responsive diode elements, also formed in an array, are each positioned to receive light energy from an adjacent scintillator element. The diode elements produce electrical signals in response to the level of generated light. That is, each diode element predominantly receives light energy from one scintillator element and generates an electrical signal corresponding primarily to the light energy in that single element. In a direct conversion detector system, semiconductor sensor elements generate the electrical signals without the need for scintillator and diode elements. In both indirect and direct conversion systems, the brightness resulting from each detector element corresponds to the level of incident radiation impinging on that element's area.
In the past, image resolution in CT systems has been a function of cost and dose efficiency. Higher resolution detectors have required denser arrays of smaller detector elements resulting in higher cost. In addition, the area surrounding the perimeter of each detector element is not responsive to x-ray energy and, as detector elements get smaller, the proportion of area on the array which becomes non-contributing to signal generation increases markedly. By way of example, the collimator may block x-rays in the perimeter region and, for an indirect conversion detector system, the reflector-filled gaps between scintillator elements are not x-ray responsive. As the non-responsive portion of a detector system increases, a higher radiation dose must be applied to the imaged object in order to sustain a desired image quality.
Detector systems may also be applied to discriminate the energy of received x-rays. Typically, several signal bins are provided for each detector element. Each bin corresponds to the received x-rays within some specified energy range. Typically, two to five bins are found in such systems. The measurement of x-ray energy provides for characterization of the imaged object's material composition. Material discrimination is possible in such a system in addition to the formation of the conventional CT image.